Replacing Rotted Top Plate in Bottom Floor Garage

We have one section of rotted top plate in our garage. There is a second story above this and I can tell it's starting to sag. Would it be best to jack up that section and remove that top plate and install a new one? Any other suggestions? The second floor does cantilever about a foot or so past this top plate. Any help would be appreciated.

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Replacing Rotted Top Plate in Bottom Floor Garage

Post some pictures, this is not going to be a smple DIY fix I believe.
What caused the plate to rot out?
I've never once seen that happen, bottom plates all the time, not top plate.
I can not even see how one could sag as you say.

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Replacing Rotted Top Plate in Bottom Floor Garage

Termite damage. This is actually at the ground level, but the way the landscape is it slopes down to the garage. In other words the landscape slopes from the front of the house to the rear. This particular wall is the front wall. This area is about four ft above the ground outside the wall. There is a concrete wall about four ft tall and then this wall on top of that. I'm a pretty experienced carpenter just want to get some ideas or suggestions. Once I get the floor supported the top plate will crumble out of there. I know I'll have some nails to clear out, but just wanted to see if anyone had any other ideas. I'm new to the forum and not sure how to post pictures but I will try in a day or so. or I could shoot them to your e mail.

Replacing Rotted Top Plate in Bottom Floor Garage

It would be best if you had someone on site that knows what there doing on this one..
Just a little tricky holding up a whole house that high off the floor.

They may be showing up there, but they had to get there somehow. Most often they came through the bottom plate, through the sheathing and or the studs. If the top splats that bad then there's more dame that you have not even seen yet.
There's going to be far more to this then just replacing a top plate, may have to do the whole wall or more.
Keep in mind your there where not to see it.

Replacing Rotted Top Plate in Bottom Floor Garage

You will need a strong temporary wall---perhaps a jack wall (this is used to do the lifting)

Removong the drywall on the ceiling in the affected area will help you install the jack wall and find other damaged wood---Joe is right---bugs eat their way through a house and seldom confine them selves to one tasty board---

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Replacing Rotted Top Plate in Bottom Floor Garage

^ + 1. A temporary wall is the very first step except where jacking is required. it is usually better to jack up sagging area and then support with temp wall before cutting out old top plate and replacing with new plate. 18 to 24" in from exterior wall is usually best place for temp wall however with the cantilever maybe outside of the structure is where to do the jacking/supporting work because top floor exterior wall ( weight ) sits out on the cantilevered section.

Replacing Rotted Top Plate in Bottom Floor Garage

Replacing Rotted Top Plate in Bottom Floor Garage

I took some pictures today, but can't seem to upload them. The sag is very minimal. I did notice there is room to build a sister wall beside the damaged wall on top of the concrete wall. I could jack the floor up just a bit and build an additional wall beside the existing wall. It's complicated to explain. I could e-mail the pictures to someone if they wouldn't mind posting them. The house is in Cumming, Ga. The garage is unfinished so everything is visible. I haven't found any termite damage anywhere else other than this top plate. I appreciate all the comments and suggestions.

Replacing Rotted Top Plate in Bottom Floor Garage

A temporary wall can be built to hold the ceiling from falling while cutting the joist and rafters loose and installing a new one. We have used a beam across the ceiling, and used jacks to raise it.
BE AWARE that each method carries significaant risk for noob. I once had a post pop loose and tear through 2 walls before hitting something and coming back into the room we were in by a new entrance. The bang deafened us for an hour and the noise was heard blocks away. It could have been fatal.

Replacing Rotted Top Plate in Bottom Floor Garage

from the pic it looks like 2 different double joist sections bearing across the top plates in your trouble area which usually indicate a wall and weight directly above or - maybe they framed a double joist every 3' in the framing plan, can't tell by pics..

otherwise a good temporary jacking system to raise the weight a tiny bit off of the plates would work enough to cut old plate out and replace with new plate ( consider using metal mending plates to splice any wall plate repair) Be very weary of temporary jacking/lifting systems as previous poster mentioned and I have seen happen, posts will kick out and go a flying.

maybe consider a temp beam put across joists 18" in from wall.

next, get two full length ( floor to ceiling) metal round post type jacks. the type that you turn and tighten at the top with a leverage bar or large wrench to make it go up. with a jacking pad placed on top of concrete slab place post jacks on pad and jack to and against temp beam above.

I'm not sure how much house weight ( rooflines, garage bearing beams etc..) you have up above where the pic does not show. I built a house one time where all of the wall headers for the second floor were placed setting on the bottom wall plates so you never really know?